Noise transmission into the interior of buildings, houses, vehicle cabins, fuselages, and other interior spaces has existed throughout human history. Reducing noise transmission for better sound quality helps enhance health, prevent sleep interference and prevent speech interference. Reducing noise transmission into enclosures can also be important for protecting people from hearing loss and complying with standards prescribed by regulators, e.g., the Environmental Protection Agency (EPA) and the Occupational Safety and Health Administration (OSHA).
Previous noise reduction techniques for glass, including but not limited to windows and windshields, have predominantly consisted of reducing the noise by a limited number of decibels over the entire hearing range (20-20,000 Hz). However, this approach typically affords little reduction in the overall noise levels, let alone low frequency noise from 20 to 200 Hz.
As seen in FIGS. 1A and 1B, previous noise reduction techniques have also included thickening of the glass window 1 (typically contained within a windowsill 3) to stiffen the window and reduce high frequency noise transmission. This can be costly, driving up the price to manufacture the glass.
Looking next at FIG. 1C, other techniques have included the introduction of air gaps within the window during the manufacturing of the glass, thereby providing double-pane windows (with a gap 39 between the two panes), whereby to mitigate high frequency vibration and thus reduce high frequency structure-borne noise. Some have even put a transparent material 40 (FIG. 1D) into the air gap to help reduce noise transmission.
Moreover, the building and construction industry utilizes R-values to rate the temperature energy efficiency of a window in relation to an interior space. The higher the R-value of the window, the higher the energy efficiency of the window. In the summer, a window with a high R-value allows less warm air to penetrate into the interior of the structure and thus helps to sustain a cool interior temperature for better energy efficiency. Similarly, in the winter, a high R-value allows less warm air to escape from a house or building, thereby helping to sustain a warm temperature for better energy efficiency.
Some glass (e.g., windows and windshields) can be modally sparse and some can be modally dense. Modally dense glass elements can typically be defined as elements that vibrate with modes (or resonant frequencies) separated by less than 40 Hz in a low frequency bandwidth between 20 and 200 Hz. In modally sparse glass elements, only one resonant frequency may be present, or two (or only a few) resonant frequencies may be present. The vibration at these resonant peaks is reduced and thus the majority of the energy is attenuated. Mass, stiffness, and damping is applied to these few peaks and noise transmission mitigated. For modally dense glass elements, many different mass, spring or damping systems can be utilized to reduce overall energy across a frequency spectrum (e.g., 20 to 200 Hz for low frequency systems and 20 to 20,000 Hz for systems of the full human hearing range).
Many types of noise can awaken people at night, distract from daily life, and affect health. For houses near busy streets, low frequency engine noise from cars, motorcycles, construction vehicles, etc. can penetrate into the interior of enclosures and awaken residents, especially when engines are revved late at night. For buildings near airports, noise from aircraft, typically in the form of blade-passage-frequency (BPF) noise, can transmit into the interior of the building. Construction next to a restaurant can make it difficult to maintain a conversation inside. Similarly, when a subway or train comes to a screeching halt, it can be nearly impossible to maintain a conversation in the interior of the car because of the brake noise. Moreover, when one is inside a vehicle and an adjacent automobile is blaring music with high bass, it can distract from the acoustics of the environment.